Referring to FIG. 1 of the drawings, the reference numeral 100 generally designates a conventional regulator having a multiplexed power supply. The regulator 100 generally comprises switches 102 and 104, and an LDO 106. Each of the switches 102 and 104 generally have a pair of back-to-back PMOS transistors Q1/Q2 and Q3/Q4, which are coupled together at their respective sources and bodies and which are controlled by control signals SEL1 and SEL2, respectively. LDO 106 generally comprises an amplifier 108, a buffer 110, and a PMOS transistor Q5.
In many applications, it is desirable to be able to select between various power supplies, which can supply different currents (i.e., 75 mA and 500 mA). Here, control signals SEL1 or SEL2 can be asserted to provide voltage V1 or V2, respectively, to the source of transistor Q5. A problem with configuration, however, is that if one of the switches 102 or 104 provides a large current (i.e., 500 mA), the switch 102 or 104 is very large to avoid a significant voltage drop across the switch 102 or 104 because of headroom requirements for some applications.
Turning to FIG. 2, an alternative regulator 200 can be seen. Regulator 200 is similar to regulator 100, but, here, switch 102 has been removed. A regulated input voltage REG from an external supply (which is filtered by capacitor C) is also applied to the node between the LDO 106 and switch 104. In this configuration, switch 104 is switched “on” (i.e., control signal SEL1 is asserted) when the external supply does not provide power to the LDO 106. A drawback, however, for regulator 200 is that use of capacitor C generally violates a sequence requirement since the voltage REG is supplied by another regulator. For example, an application may require that the voltage REG remain logic low or “0” when switch 104 is in an “on” state, which means that regulator 200 would violate this requirement.
Therefore, there is a need for an improved regulator.
Another example of a convention circuit is U.S. Patent Pre-Grant Publ. No. 2009/0039947.